Quest's companion diagnostic confronts Tysabri brain infection risk

It's a vexing problem that certain drugs with enormous benefit can also carry significant health risk.

Case in point: Some patients who take the multiple sclerosis drug Tysabri experience improved health, but face an increased risk of contracting a rare and potentially fatal brain infection known as PML, which grows from the oddly named John Cunningham virus. The FDA on Jan. 20 approved Quest Diagnostics' ($DGX) Stratify JCV companion diagnostic test in order to reduce the risk for those patients who are particularly susceptible.

Stratify is the first test of its kind designed to identify the virus in patients who take Tysabri. Developed by Biogen Idec ($BIIB) and Elan ($ELN), Tysabri suppresses the immune system and as a result, puts patients who have the otherwise harmless virus at increased risk of developing progressive multifocal leukoencephalopathy, which causes brain swelling. Quest had filed a de novo classification for the test, and worked closely with Biogen Idec and Elan to develop it, the company said. Quest's Focus Diagnostics laboratory will offer the test in the United States. Separately, the FDA has revised Tysabri's label with new language explaining that patients who test positive for the John Cunningham virus are more likely to develop PML, The Associated Press reports.

About 400,000 people have MS in the United States alone, and 50% to 60% of MS patients have the John Cunningham virus, Quest estimates.

Tysabri is a miracle injectable drug for MS patients, but created serious alarm shortly after its 2005 launch. As the AP reports, FDA pulled the drug after at least three patients taking it developed PML. A year later, regulators allowed Tysabri back to the market but placed limitations on its prescription and distribution, to doctors and pharmacists who took part in Biogen Idec's distribution program. Still, the drug is a healthy seller now for both Biogen Idec and Elan.

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